1. Field of the Invention
This invention relates to a fastener for securing a bone plate to surrounding bone, for example when replacing a portion of the cranial vault removed during a craniotomy for a neurosurgical procedure. The invention also relates to an instrument for securing two threaded members to each other. The instrument may be used during neurosurgical procedures, or as a mechanical tool in many other contexts.
2. General Discussion of the Background
A craniotomy is a surgical procedure in which a portion of the cranial vault is removed or folded back in a flap to permit surgical access to the cranial contents (such as the brain). To perform the craniotomy, several burr holes are drilled through the skull. The number and position of these holes varies depending on the shape of bone to be removed. For example, three burr holes are drilled at corner points if a triangular bone flap is desired. The burr holes are then connected by osteotomy cuts, for example using a Gigli flexible saw which is passed internally between the burr holes. The saw is then oscillated back and forth to cut the skull along a line of separation (defined by the connecting osteotomies) connecting adjoining burr holes. The bone cover is subsequently lifted off the underlying dura mater to expose the brain. The bone cover may either be completely removed from the surgical site, or folded back in a flap along an uncut edge of the flap.
After completion of the operation, the bone cover must again be fixed in its original position to protect the underlying brain. Conventional craniotomy closure requires that holes be drilled in the bone plate and surrounding cranium along the osteotomy lines, and stainless steel wire or silk sutures are then passed through the holes to retain the plate in place. Unfortunately, drilling the holes is time consuming and potentially dangerous, because of the risk of introducing infection. The sutures are also unstable and prone to breakage. Suture instability or breakage can lead to dangerous movements of the cranial plate against the brain, with pathologic sequelae similar to a depressed skull fracture. The sutures closing the osteotomy lines are not aesthetically pleasing, because they can leave irregularities in the overlying surface of the face or scalp. This irregularity is particularly unsightly if the surface of the bone plate is not held substantially co-planar with the surrounding bone.
Various fixtures have previously been proposed for securing the bone cover to the surrounding cranium. U.S. Pat. No. 5,201,737 discloses a flexible plate having a plurality of vanes with holes for receiving bone screws. The plate is placed over a cranial burr hole and adjoining osteotomy lines to provide external fixation of the bone cover to the surrounding cranium.
Other external bone plates are shown in U.S. Pat. No. 4,651,724; U.S. Pat. No. 4,923,471; U.S. Pat. No. 5,139,497 and U.S. Pat. No. 5,372,498. All of these plates are designed for external application to fractured bones, and require placement of a plurality of screws through the plates. As with the plate in U.S. Pat. No. 5,201,737 discussed above, placement of multiple screws through the plates is time consuming, predisposes to catastrophic infection, and is difficult to remove once in place.
A variety of fixation devices are also known for fusing fractured bones. An example of such a device is U.S. Pat. No. 2,511,051, in which an externally threaded stud screws into an internally threaded shank. Movement of the stud into the shank is guided by an hexagonal wrench that is inserted through the shank into a countersunk receptacle on the tip of the threaded stud.
U.S. Pat. No. 3,875,936 shows an attachment for replacing a trochanteric head to the femur by providing a barbed shear washer between the femur and trochanteric head. U.S. Pat. No. 5,098,433 uses a winged compression bolt for fusing fractured bones. U.S. Pat. No. 5,196,016 and U.S. Pat. No. 5,433,719 discloses fixation pins or screws for retaining bone fragments against one another.
In spite of the use of a variety of fasteners in orthopedic and neurosurgical procedures, improved techniques are still being sought to secure a cranial cover to the surrounding cranium following a craniotomy. Improved methods and devices for securing the fasteners to the skull are also disclosed.
Accordingly, it is an object of the present invention to provide a fastener that is especially suitable for closure of craniotomies.
Yet another object is to provide such a fastener that can be quickly and efficiently installed, and which is capable of easy removal in the event that subsequent intracranial access is required for another neurosurgical procedure.
Yet another object of the invention is to provide such a fastener that avoids the aesthetic drawbacks of prior fasteners, such as large indentations in skin overlying the craniotomy.
It is another object of the invention to provide such a fastener, and an instrument for manipulating the fastener, that allows it to be easily inserted and removed, yet which provides a potentially permanent and reliable fixation of the cranial cover, thereby avoiding the potentially catastrophic neurological consequences that can result from dislodgement or depression of the cranial cover.
Finally, it is an object of the invention to provide an instrument that is useful for engaging first and second members to each other, particularly when the engagement must occur on opposite surfaces, and one of the surfaces is not easily accessible.
These and other objects are achieved by the bone cover fastener of the present invention, which has internal and external fastening members. At least one of the fastening members has a conforming surface that conforms to a curved surface, such as the inside surface of the cranial vault. In other embodiments, both the internal and external fastening members have conforming surfaces, such that a surface of the internal fastener conforms to the internal surface of the cranium, while the external fastener also has a surface that conforms to the external surface of the cranium.
In one embodiment, there is a connector on the internal fastening member and a connector on the external fastening member, and the connectors cooperatively hold the internal and external fastening members in a fixed relationship that fixes the bone cover in a defect from which the bone plate has been removed. The conforming surface may be a curved surface of the internal fastening member that seats against the inner face of the cranium. In particularly disclosed embodiments, the internal and external fastening members are curved plates that respectively conform to the internal and external curvature of the skull. Alternatively, the conforming surface may be provided by flexible struts that deform into a curved configuration as the fastener is tightened.
In other embodiments, a tab is provided on one of the fastening members to prevent relative rotation between the fastening member and the cranium. In particular, the tab is a relatively flat member that extends away from the internal fastening member a sufficient distance to be retained between the bone plate and the remaining cranium. The tab is sufficiently thin that it fits within the gap between the bone plate and surrounding cranium. When the external fastening member is rotated relative to the internal fastening members to interconnect threaded connectors, rotation of the internal fastening member is opposed by the tab which is retained within the gap formed by the cranial osteotomy incision.
One of the connectors may be an elongated, externally threaded stud that projects from the first fastening member, while the second connector may be an elongated internally threaded collar that projects from the second fastening member. The stud is rotationally threaded into the collar, with the collar extending between the fastening members. The collar fits through the burr hole of a craniotomy incision, and each fastening member is wider than the collar and the burr hole to provide clamping surfaces above and below the burr hole that engage the internal and external surfaces of the bone cover and surrounding cranium. Screwing the threaded stud into the internally threaded collar brings the opposing fastening members closer together, and tightens them against the internal and external surfaces of the bone cover and surrounding cranium. The fastening members overlap margins of the burr hole and adjoining osteotomy lines to securely fix the bone cover to the surrounding bone.
In a disclosed embodiment, one of the fastening members is a base which includes a disc having a flat inner face and a flat outer face. A plurality of raised barbs extend from the inner face of the base to provide frictional engagement between the disc and bone. The other fastening member is a cap with an outer face that may be flat or convex, and a recess is provided in the outer face for engaging a drive member that rotates the cap. An externally threaded stud projects from an inner face of the base and an internally threaded collar projects from an inner face of the cap. The stud is approximately as long as the collar. The stud may be screwed partially or entirely into the collar, depending on the thickness of the bone cover. Hence the fastener has the versatility to be used in different locations of the skull, where bone thickness varies.
The fastener is used in a method for fixing a bone plate, such as a cranial cover, in a bone defect, such as a craniotomy opening. The bone plate has opposing internal and external surfaces that are to be held in position substantially co-planar with internal and external surfaces of surrounding bone. The method provides the steps of placing the base and cap of the fastening member on opposing internal and external surfaces of the bone plate, with the collar projecting into the hole, and a portion of the base and a portion of the cap overlapping the border of the junction between the bone plate and surrounding bone. The base and cap are then rotated into threaded engagement with one another, until the cap and base tightly engage opposing surfaces of the bone plate and surrounding bone to clamp the bone plate in place. In particularly preferred embodiments, the fastener is placed through a craniotomy burr hole, with the cap and base covering and closing the burr hole.
In yet another embodiment of the invention, a fastening instrument is provided to secure first and second members to each other. The instrument includes a sleeve, a shaft or rod that slides within the sleeve, an attachment mechanism on the sleeve for engaging the first member to the instrument, and an engagement structure on the shaft that engages the second member and pulls it into engagement with the first member. In more particular embodiments, the sleeve includes a handle extending out from the sleeve, and the rod includes a handle that allows the rod to be rotated and reciprocated relative to the sleeve. There is a threaded tip on the rod that engages an internally threaded collar of the first member to screw the first and second member to be moved into engagement with each other.
In a more particular embodiment, the instrument includes a sleeve that has a handle (such as a rod or disc) extending from the sleeve. A rod slides within the sleeve, and has a threaded tip at a distal end of the rod, and a handle (such as a disc or curved hand grip) near a proximal end of the rod. The handle extends perpendicularly to the rod, and allows an operator to both rotate the rod and move the rod axially within the sleeve. A distal tip of the sleeve that surrounds the threaded rod has a plurality of locking members or prongs that extend from the tip of the sleeve and fit into complementary receptacles on one of the fasteners to lock the fastener to the tip of the sleeve, for example by frictional engagement of the prongs to one of the fasteners.
The instrument is designed to engage two threaded fasteners to each other. The fastener includes a cap and a base, and a receptacle in the cap that engages the prongs on the sleeve to lock the cap to the sleeve. The cap also includes an opening through which the rod can slide. The base of the fastener includes a post having both internal and external threads, and the threaded tip of the rod has threads that are complementary to the internal threads of the post. The cap has a collar with internal threads that are complementary to the external threads on the post.
The fasteners are attached to each other by first engaging the fastener cap to the sleeve of the base by introducing the prongs into the receptacles on the cap. The rod is then extended through the cap, and the base secured to the rod by threading the threaded tip of the rod into the internally threaded post of the base. The sleeve is then axially advanced over the rod until the external threads of the base post abut the internal threads of the cap collar. The rod and sleeve are then rotated relative to each other (for example by rotating the shaft) to screw the externally threaded tip of the post on the base into the internally threaded collar of the cap. The threaded rod tip is then unscrewed from the internally threaded post of the base. The connected fasteners may then be disengaged from the instrument, for example by pulling the instrument away from the fasteners to disengage the locking member from the fasteners. Alternatively, axial movement of the rod towards the fasteners brings the threaded rod tip into abutment against the internal threads of the post, and pushes the fasteners off the locking members to disengage the fasteners from the instrument.
The instrument of the present invention can be used with many different types of fasteners, in many situations where two fasteners must be secured to each other in a difficult to reach location. The instrument is particularly useful when the fasteners are to be secured to opposite faces of a barrier that inhibits access to both faces of the barrier. One particular fastener with which the instrument can be used is the bone cover fastener of the present invention.